Abstract

In most theoretical and experimental studies of multi-component complex fluids, it is implicitly assumed that if a fluid is well-mixed at rest, it will remain well mixed in flow. To leading order, this may be a reasonable first approximation for many materials and many applications. However, there is an abundance of experimental evidence to show that well-mixed polymer melts and solutions can fractionate or even demix in the presence of a sufficiently strong shear flow. These phenomena are scientifically interesting but also industrially pertinent; millions of pounds of polymers are processed every day, and unexpected compositional inhomogeneity can negatively impact operating conditions and/or product quality.

Theoretical studies on compositionally inhomogeneous flows of polymer melts and solutions have typically made use of two-fluid models, wherein the molecular details of each species are coarse-grained into a set of superimposable continuum fluids. The relative motions of the two fluids are restricted by frictional drag, and the collective motions of the two fluids are restricted by incompressibility of the overall velocity field. In the present talk, we employ a two-fluid model of semi-dilute entangled polymer solutions to consider whether the linear and nonlinear dynamics of a shear induced demixing instability. We will present a summary of results for linear stability, nonlinear evolution, and provide a visual comparison to the results of a thermodynamically driven model of demixing. Finally, we make use of an asymptotically limiting version of the model to determine whether shear induced demixing resembles a thermodynamically driven demixing instability, as has often been assumed. We will show that, surprisingly, the answer to this question depends on the nature of the imposed deformation (e.g. steady vs. unsteady flow).